Chronic pain differs from acute pain in that it serves no protective or biological function. While clinical symptoms differ between acute and chromic pain, we are only now beginning to' understand some of the underlying neurochemical and functional differences that occur at the level of the spinal cord. On the other hand, very little is known regarding pain-induced alterations that occur at supraspinal levels. The present proposal addresses this issue and represents a continuation of previous work in our laboratory in which the effects of chronic nociception on the neurochemistry of midbrain neurons associated with regions capable of producing antinociception are analyzed. The moor goals of the proposed research are 1) to elucidate how the neurochemistry of chronic pain differs from or is similar to that of acute pain; and 2) to better understand the function and chemical organization of descending antinociceptive circuits. The main hypothesis to be tested in this proposal is that activity.dependent alterations occur at three different but interrelated levels within brainstem neurons that participate in nociceptive pathways: a) peptide and GAD mRNA concentrations, b) peptide and GABA release, and c) postsynaptic receptor mRNA concentration. The experiments that will address these goals and this hypothesis are defined by three interrelated specific aims: Specific aim #1: To define the anatomical projections of midbrain and lateral hypothalamic neurons whose neuropeptide or GAD mRNA levels are altered by nociception. This will be accomplished using a combined retrograde transport-in situ hybridization procedure together with quantitative image analysis to determine if selected peptide mRNAs or GAD mRNA levels are altered in neurons located in the PAG, nucleus cuneiformis, microcellular tegmental nucleus or lateral hypothalamus, regions that participate in antinociceptive circuits. Specific aim #2: To determine if exposure to short-term or long- term chronic nociception alters the concentration of extracellular neuropeptides and GABA in the periaqueductal gray, the ventromedial medulla and the lateral hypothalamus. This will be accomplished using in vivo microdialysis and HPLC or radioimmunoassay to analyze the relative concentrations of selected neuropeptides and GABA in the extracellular fluid of experimental (sciatic nerve ligatures) and control animals. Specific aim #3: To ascertain whether chronic pain modulates the expression of neurotensin, substance P, delta opioid, mu opioid, kappa opioid or GABA-A receptor mRNA in neurons retrogradely labeled from the ventromedial medulla or periaqueductal gray. This will be accomplished using retrograde labeling together with in situ hybridization analysis of the mRNAs for these receptors. The results of these studies will provide new data regarding the neurochemical alterations that occur in brainstem neurons following exposure to acute versus chronic pain and will define the anatomical connections of these neurons. Such data are valuable for the development of better therapeutic approaches to control acute and chronic pain.